>From: John Brownlow <john@pinkheadedbug.com>
>this is nuts. loads of people, me included, edit by the numbers. The whole
>point of an ICM profile is so that the same RGB values display the same on
>different profiled devices.
Sorry to tell you, but you are *so* wrong. RGB values mean diddly squat.
They are just a measurement of the amount of current in a scanner ccd or the
output voltage of a video card. The ICM profile provides a description of
the transfers that are required to turn the RGB values into proper
colimetric tristimulus values (i.e. XYZ). That then enables transfers
between colour spaces to happen.
(extreme example) You could have a scanner that mapped mid grey at 20,20,20
and a monitor that mapped it at 230,230,230. The ICM Profile describes this
and allows colour management systems to convert from one colour space to
another, and the RGB (device-dependant) values change accordingly, depending
on which space you are in, but the device-independant colour space values
(XYZ, LAB, X'Y'Z' etc) would not change.
>Incidentally, 18% grey means that the card REFLECTS 18% of the light. So it
>is a relative grey (relative to 100% reflectance of the incident light).
>Not
>to 'pure' white, whatever that may be.
It all gets majorly complex when talking about scanning film, as the CCDs
are only registering a value that they see between their black and white
points. The first problem with doing all of this is 'has the film been
processed exactly the same way as the one the profile was generated from'
and 'is the exposure totally spot on?
The answer to both of them is generally no :-)
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